This should block bug 1275725, since we should ship something like this without being able to blacklist drivers.
It should be sufficient to just add a normal blacklist entry. (similar to FEATURE_WEBGL_OPENGL)

Fairly simple addition of a gfxConfig feature for managing surface sharing. All places the feature is checked can reasonably expect the GLX library to be initialized.
It's currently not very necessary to ensure that this feature is synchronized between content and chrome processes, but it might be a good idea later if we decide to disable the feature at runtime due to a failure in the compositor's texture host.